550 JAMES CLERK MAXWELL. 



the actual connection of the parts of the electro-magnetic field. 

 The problem of determining the mechanism required to establish 

 a given species of connection between the motions of the parts of 

 a system always admits of an infinite number of solutions. Of 

 these some may be more clumsy or more complex than others, 

 but all must satisfy the conditions of mechanism in general. 



The following results of the theory, however, are of higher 

 value : 



(1) Magnetic force is the effect of the centrifugal force of 

 the vortices. 



(2) Electromagnetic induction of currents is the effect of 

 the forces called into play when the velocity of the vortices is 

 changing. 



(3) Electromotive force arises from the stress on the con- 

 necting mechanism. 



(4) Electric displacement arises from the elastic yielding of 

 the connecting mechanism. 



In a paper entitled "A Dynamical Theory of the Electro- 

 magnetic Field," read before the Royal Society on December 

 8, 1864, Maxwell deduced all the above results by purely 

 mechanical reasoning, only assuming the existence of a 

 medium capable of receiving and storing up potential and 

 kinetic energy, and therefore capable of doing work in " re- 

 covering from displacement in virtue of its elasticity," while 

 the parts of the medium are connected by " a complicated 

 mechanism capable of a vast variety of motion, but at the 

 same time so connected that the motion of one part depends, 

 according to definite relations, on the motion of other parts, 

 these motions being communicated by forces arising from 

 the relative displacements of the connected parts, in virtue 

 of their elasticity." For the existence of such a medium we 

 have evidence independent of electrical actions. With regard 

 to the mechanism no attempt is made in the paper to give to 

 it any definite constitution. This paper has been regarded as 

 Maxwell's greatest contribution to electrical science, but most 

 of the results obtained in it have been already mentioned. 



The following is a good specimen of Maxwell's humor- 

 ous irony, of which there are many samples in his scientific 

 works. He is discussing certain developments by Bernhard 

 Riemann Lorenzo, of Weber and Neumann's theory of Electro- 



